Just my two penny worth... new to Zs but not new to mechanics....
Parts don't have to be new, they have to be in a suitable condition to pass an MOT.
Something like this looks okay (assuming its the right one) degrease it, wire brush on a drill, rust convertor paint, a couple of coats of chassis black / wax spray in the box, and its ready to be fitted
NISSAN 350Z MK1 Z33 2003 - 2009 FRONT SUBFRAME 76386 | eBay
Depends on your experience / tools / work area if you could DIY a swap. I reckon it would take me a long weekend for the front if I did it myself
Rear is aluminium, as post above and this clearly shows, your MOT inspector has had a mare with that one
2005 NISSAN 350Z V6 Z33 3.5 PETROL MK1 3DRS COUPE REAR SUBFRAME (VQ35DE) | eBay
W brace and kidney brackets are very easy swaps, it would take me no more than a couple of hours if I did it myself
My best guess a garage would want maybe 2 days?